Telecommunications is a field which has been rapidly evolving over the past twenty years, fueled in part by the progressively increasing popularity of technology such as cellular telephones, facsimile machines and computer communications that use the Internet. Due to these growing new technologies, there has been a progressively increasing demand for telecommunications equipment with greater information-carrying capacity, which in turn has created a progressively increasing focus on the use of optical signals to effect communications.
Currently, when existing systems need to process an optical signal, they typically convert the optical signal into an electrical signal, then process the electrical signal, and then convert the processed electrical signal back into an optical signal. This greatly delays propagation of information through the system, and is expensive because it increases the complexity of the system. In order to avoid these types of problems, there is a progressively growing demand for equipment that can directly process optical signals, without temporarily converting them into electrical signals.
On existing type of device which can directly process optical signals is an optical add/drop multiplexer (OADM). The multiplexer has a primary input, an express output, an add input, and a drop output. The primary input receives an optical signal having several components at respective different wavelengths. The multiplexer extracts one of these components signals and supplies it to the drop output, and routes the remaining signal components to the express output. Simultaneously, the multiplexer can accept at its add input an add signal component at the predetermined wavelength, which it supplies to the express output.
While existing multiplexers of this type have been generally adequate for their intended purposes, they have not been satisfactory in all respects. In this regard, a small portion of the energy of the signal component that is to be sent to the drop output may leak through to the express output, where it can cause interference with the add signal component, due to the fact that they have the same wavelength. In a high-performance application, the isolation at the express output of the drop signal component with respect to other signal components may need to be as high as ˜23 dB, or even higher, but existing approaches that can achieve this level of performance typically involve a higher number of parts and a higher cost than is desirable. Another problem is that a small portion of the add signal may leak through to the drop output, where it can interfere with the drop signal component.